(a) Field of the Invention
The present invention relates to a semiconductor memory device and a method for writing data therein and, more particularly, to an electrically erasable and programmable semiconductor memory device and a method for writing data therein.
(b) Description of the Related Art
An electrically programmable ROM is one of nonvolatile semiconductor memory devices into which data are electrically written. One conventional electrically programmable ROM (hereinafter called "EPROM") is described in Japanese Patent Laid-Open Publication No. 62(1987)84496).
Recently, an electrically erasable and programmable ROM (hereinafter called "EEPROM") such as described in Japanese Patent Publication No. 4(1992)-80544 has come to public attention. In an EEPROM, a dispertion or variation is inevitably produced in threshold for electrically erasing data therefrom among memory transistors. In order to eliminate the variations, there has generally been adopted an operation off a so-called pre-erasure writing in which all of the memory transistors in a memory cell array undergo a write operation before their erasure so that the thresholds of the memory transistors for erasure are made uniform among the memory transistors.
However, when the pre-erasure write operation is carried out in a conventional method, channel current flows in each of the memory transistors during the write operation. Therefore, if data are simultaneously written into all the cells, an extremely large current is consumed in the memory cell array. Accordingly, there has been adopted a method in which data are serially written on a 1-byte (8-bit) or 1-word (16-bit) basis for the pre-erasure write process, taking into consideration of the restriction imposed upon the total current consumption. Therefore, a large amount of time is required until all of the memory transistors have undergone the write operation before their erasure. This hinders the semiconductor memory device from operating at a higher speed.